In general, alloy materials such as carbon tool steels, high-speed steels, and high-carbon martensitic stainless steels are widely used as blade materials of cutters, for example, in addition to knives for meals and foods, cooking knives, and camping knives (field service knife, outdoor knife); scissors, ice picks, cutters for food machines, cutters for cutting frozen foods, paper cutters, cutters for perforating a plastic package of, for example, tablets, cutters for medical use (surgical knives, chisels, and scissors), and cutters for cutting plastics. Titanium alloys are also used as a material of cutters for special purposes.
Although ingots prepared by melting raw materials and solidifying the molten materials are generally used as the alloy materials for constituting the above cutters, alloy materials produced by powder metallurgy are also partly used. Except for the above titanium alloys for special purposes, as will be described later, knives, i.e., cutters (cutting tools) composed of the above alloy materials are generally produced as follows: A steel blank is formed to have a knife shape. The formed body is then subjected to heat treatment so that carbides having high hardness are finely dispersed and precipitated in the martensitic structure. This process provides the knives i.e., cutters, with the hardness required for the cutters.
For example, Japanese Unexamined Patent Application Publication No. 10-127957 discloses a knife for meals as an example of the above cutters. The knife is produced by welding a blade part composed of an austenitic stainless steel including predetermined contents of C, Si, Mn, P, S, Ni, Cr, Mo, N, and the balance Fe, and in addition, having a Vickers hardness (Hv) of at least 450 with a metallic grip part. In addition to the above example, cutters composed of an iron-based alloy material such as a martensitic stainless steel have been also widely used.
A method for producing a knife will now be specifically described with reference to an example of a knife. The knife is composed of an iron-based alloy material, for example, a martensitic stainless steel, which is the most versatile and in widespread use.
FIG. 8 includes perspective views showing a process for producing a known knife composed of a stainless steel. The knife composed of a stainless steel is generally produced by processing a plate 1 composed of a martensitic stainless steel, which can be hardened by quenching. When such an iron-based alloy material is used, the plate 1 is annealed in advance to facilitate the machining (machine work). Subsequently, the plate 1 is cut by punching to form a formed body 3 having a predetermined shape. This machining provides the cutter shape at normal temperature. The plate 1 is processed by cutting, grinding, and polishing or by hot forging to form a near net shape of the cutter, thus forming a cutter blank (tool raw material) 4. At the handle part (grip portion), grip-fixing holes 2 are formed by, for example, a drilling machine.
Subsequently, the processed cutter blank 4 is heated up to the predetermined quenching temperature, kept at the temperature for the predetermined time, and then quenched to provide the predetermined hardness. In general, carbon steels for cutters are heated in air, and other metallic materials are heated in a vacuum, in an inert gas atmosphere, or in a non-oxidizing atmosphere. The carbon steels or the other metallic materials are kept in an adequate temperature range, which depends on the kind of the alloy material, for the predetermined time, and then hardened by quenching.
The quenching temperature is different depending on the kind of the material. The quenching temperature of carbon steels is from 700° C. to 900° C. and that of stainless steels is from about 950° C. to about 1,100° C. The optimum temperature range is from 40° C. to 50° C. Water quenching, oil quenching, and forced air-cooling are used for the quenching according to the kind of the material.
A deep cooling (low-temperature treatment), i.e., a sub-zero treatment may be performed according to need. In the sub-zero treatment, a sample is submerged into a cold material at a low temperature such as liquid nitrogen or dry ice to cool the sample at a low temperature of 0° C. or less. This sub-zero treatment causes the martensitic transformation of the retained austenite in the stainless steel structure and prevents the aging (secular change) of the cutters.
However, because of the high hardness, the cutter blank 4 hardened by quenching has poor toughness and is brittle without further treatment. Unfortunately, such a cutter blank 4 often causes chipping and cracking of the blade. In order to prevent this problem, the cutter blank 4 is then tempered (tempering treatment). The conditions for tempering are different depending on the application of the cutter and the kind of the material. In general, carbon steels are tempered in a temperature range of about 160° C. to about 230° C., and stainless steels are tempered in a low temperature range of about 100° C. to about 150° C. to provide the predetermined toughness.
Subsequently, in order to remove an oxide film and a discolored part generated by heat treatments such as the quenching and the tempering, the surface of the cutter blank 4 is polished for finishing, thus preparing a blade body 5. In some cases, the cutter blank 4 is further polished to form a mirror finished surface. This process adjusts the color tone and the luster of the blade body to enhance the decorative and aesthetic properties. Furthermore, a grip 6 is attached to the blade body, and the blade edge is finally sharpened to complete a knife 7 as a cutter product (cutting tool).
Functional characteristics of the cutter required from the standpoint of users generally include items such as the cutting quality (sharpness), the superior blade durability (hardness, toughness), rusting resistance, the ease of sharpening, and decorative properties (luster, color tone). Characteristics of the cutter required from the standpoint of cutter manufacturers include, for example, machinability (the ease of cutting, the ease to produce a mirror finished surface, and the processable temperature range to produce a cutter by forging) and the ease of heat treatment (the temperature range of heat treatment, the critical quenching speed, the atmosphere during heat treatment, and less quenching distortion and quenching crack). In addition to the above required characteristics, knives for frozen foods and knives used in cold areas essentially require the cold resistance that prevents low-temperature embrittlement.
Accordingly, the workability to form a knife shape, the ease of heat treatment, and the ease of finish machining of the surface such as a mirror finished surface are important factors rather than the cost of steel blanks itself in order that knife manufacturers can decrease production cost. For the knife users, on the other hand, in addition to corrosion resistance, the cutting quality, and the ease of sharpening; decorative properties wherein a metallic luster has a high grade feeling are also an important factor. Furthermore, in knives used in very special purposes such as knives for frozen foods, cutters for food machines, and knives used in cold areas, toughness at low temperature is important. In knifes for meat, less attachment of the tallow is important. In a magnetic field environment, it is important that cutters are not magnetized. In surgical knives and the cutters for food machines, it is important that the cutting quality is not deteriorated by sterilization at high temperatures.
However, materials that can satisfy all the above characteristics required for the cutters are not in practical use. In reality, cutters are produced with materials that may sacrifice any of the above characteristics, and such cutters are unsatisfactorily obliged to use under the present situation. For example, when priority is given to the blade durability and the cutting quality, carbon tool steels are selected as the material. On the other hand, when priority is given to corrosion resistance, martensitic stainless steels are selected. Unfortunately, the former carbon tool steels readily rust and are significantly deteriorated with age. Therefore, at present, cutters composed of the latter martensitic stainless steels are the main stream on the market. However, in terms of the blade durability and the cutting quality, cutters composed of the latter martensitic stainless steels are somewhat inferior to those of the former carbon tool steels. In any case, all required characteristics are not satisfied.
As described above, for example, martensitic stainless steels having improved main characteristics such as the blade durability and the cutting quality are on the market as the material for cutters. However, these alloy materials generally have a bad machinability. In addition, these alloy materials require a strict and precise control of the heat treatment temperature to achieve the desired characteristics. As a result, these alloy materials require advanced techniques and a large amount of labor for operation management of the production equipment. Unfortunately, these problems significantly increase the production cost of the cutters such as knives.
Even though known cutters such as knives are composed of stainless steels, the stainless steels are martensitic alloys, which are significantly inferior to austenite alloys in terms of corrosion resistance. After the cutters are used; sweat, saline water, and blood are attached to the cutter. When maintenance cleanings are neglected, such attachments and leaving without further treatment drastically deteriorate the cutting quality within a short period of time and often generate rust. Unfortunately, the maintenance, the renewal, and the management of the known cutters are complex. In particular, for example, in 14Cr-4Mo stainless steels, which are now widely used as steels for high grade knives, the contact with saline water readily causes pitting corrosion. Therefore, the above stainless steels have a short durability (lifetime) and a problem in view of food sanitation.
Furthermore, since known cutters composed of iron-based alloys such as stainless steels are composed of a magnetic material, it is difficult or impossible to use such cutters under an environment including a magnetic field, for example, in a medical facility, e.g., an MRI. Therefore, although ceramics cutters are used for this purpose, such ceramic cutters have a poor cutting quality, compared with metallic cutters. Unfortunately, precise cutting operations are difficult to achieve.
Furthermore, a flange-shaped hilt is attached to, for example, outdoor knives for fear that users may carelessly touch the blade edge part. In order to attach the hilt to the knives, the blade body is heated to melt a brazing material, i.e., binder. Unfortunately, this process blunts the heated part and significantly decreases the hardness in the heated part and the peripheral part thereof. In particular, the abrasion of the blade edge drastically deteriorates the cutting quality. In addition, cutters that require sterilization, for example, cutters for food machines and surgical knives, are repeatedly sterilized by heating. However, such cutters and knives are obliged to be sterilized at a low temperature, or, in some cases, to be sterilized at a low temperature with medical agents for fear of blunting of the heated part and decreasing in the hardness. Unfortunately, the cutters and knives are insufficiently sterilized.
In order to solve the above problems and technical challenges, it is an object of the present invention to provide a cutter composed of a Ni—Cr alloy, in particular, produced with a superior workability and by a significantly simplified process, having a low deterioration in the hardness even when heated in use, having excellent corrosion resistance and low-temperature embrittlement resistance, and satisfactorily maintaining the cutting performance for a long time.